Method of exhausting silicon oxide

ABSTRACT

A monocrystal pulling apparatus according to the Czochralski technique, provided with a flow controller which guides a carrier gas supplied from the top of a pulling cheer to the surface of a melt of a material forming the monocrystal and exhausts the silicon oxide vaporizing from the surface of the melt to the outside of the pulling chamber and which surrounds the pulled monocrystal near the surface of the melt and is provided partially inside a crucible, wherein the flow controller has a tubular portion which has an outer diameter smaller than the inner diameter of the crucible and extends substantially perpendicularly along the direction of downward flow of the carrier gas, a constricted diameter portion which constricts in diameter from the bottom end of the tubular portion and forms a bottom gap with the pulled monocrystal, and an engagement portion which projects out from the top of the tubular portion and forms a top gap at the outer circumference of the tubular portion of the flow controller by supporting the flow controller partially in the pulling chamber. As a result, a first flow path through which the carrier gas flows toward said bottom gap is defined between the inside of the tubular portion and the pulled monocrystal, a second flow path is defined comprised of a flow path of the carrier gas passing through the top gap and a flow path of the carrier gas passing from the first flow path through the bottom gap and then passing between the surface of the silicon melt and flow controller. The silicon oxide is exhausted together with the carrier gas through the second flow path to the outside of the pulling chamber.

This application is a divisional of copending application Ser. No.08/187,551, filed on Jan. 28, 1994, now allowed, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a monocrystal pulling-up system whichpulls up and grows a monocrystal of silicon etc. by the Czochralskitechnique (hereinafter referred to as the "CZ technique"), in particularrelates to a monocrystal pulling-up system which can pull a largediameter and high quality monocrystal while controlling the oxygenconcentration (density) by providing a carrier gas flow controller.

The present invention also relates to a method of exhausting siliconoxide(SiO) produced from the melt silicon in a quartz crucible to theoutside of a pulling chamber by controlling the flow of the carrier gas.

2. Description of the Related Art

To produce a silicon monocrystal by the CZ technique, polycrystallinesilicon and the necessary dopant(s), for example, P, B, Sb, As, areinserted into a quartz crucible provided rotatably at a bottom of apulling chamber, the chamber is evacuated to a vacuum, then a heaterarranged around the quartz crucible is used to melt the polycrystallinesilicon and the dopant(s). A carrier gas is then passed from an upperportion of the chamber to the quartz crucible. At the same time a seedcrystal (starting crystal) attached to and supported by a chuck on apulling shaft is immersed in the melt silicon in the quartz crucibleunder conditions of a vacuum of 10 to 20 Torr. Then the pulling shaft ispulled up at a predetermined speed while relatively rotating it withrespect to the quartz crucible.

U.S. Pat. No. 4,330,362 discloses a pulling system provided with amember (hereinafter called a "heat cap") comprised of a material able toreflect ultraviolet rays above the crucible so as to partially cover thecrucible and the melt silicon in the crucible, to thereby block theradiant heat from the melt surface, promote the formation of themonocrystal, and raise the pulling speed and to keep down theconcentration of carbon in the monocrystal.

When pulling a silicon monocrystal using the above-mentioned pullingsystem, however, there are the following disadvantages.

First, the above heat cap can be expected to a certain extent to havethe effect of controlling the flow of the carrier gas, for example,argon gas(Ar), being passed to remove the silicon oxide(SiO) producedfrom the melt and efficiently eliminating the silicon oxide depositingon the inner wall of the top end of the crucible, but silicon oxide endsup depositing and condensing on the top of the heat cap itself. This isa drawback in that it would fall onto the melt silicon surface and thusobstruct the formation of the silicon monocrystal. This is believed tobe because the heat cap has as its main object the literal blocking ofultraviolet rays and is not designed with the intention of control ofthe flow of the argon gas.

Second, the melt surface near the inner peripheral wall of the quartzcrucible ends up being covered by the heat cap, so an operator cannotvisually inspect the melt surface from a peephole etc. provided inthe-pulling chamber. As a result, there is the disadvantage that it isnot possible to quickly deal with any heat deformation of the top end ofthe crucible, recrystallization or deposition of silicon near the innerperipheral wall of the crucible, or any other disadvantages when theyoccurred.

In addition, there are the following disadvantages when trying to pull asilicon monocrystal for use for the manufacture of a large diameter VLSidevice, for example, a diameter of 6 inches, 8 inches, or more.

With a large diameter crystal, the most important thing is the controlof the oxygen concentration (density). This is generally classified inthe manufacturing process of an LSI device into high oxygen of, forexample, 1.55×10¹⁸ atoms/cm³, medium oxygen of, for example, 1.35×10¹⁸atoms/cm³, and low oxygen of, for example, 1.15×10¹⁸ atoms/cm³. Further,in some cases, extremely high oxygen and extremely low oxygen aredemanded and such classifications are selectively used. For example,when using the intrinsic gettering technique utilizing oxygenprecipitation, much use is made of monocrystals from a high oxygen to amedium oxygen concentration. On the other hand, when strength andreduction of lattice faults are required, much use is made ofmonocrystals from a medium oxygen to a low oxygen concentration. Thus,it is necessary to control the variation in the oxygen concentrations inthe axial direction of the crystal and in the silicon wafer surface sothat the oxygen concentration of the pulled monocrystal becomes in thedesignated narrow range.

The "oxygen in the crystal" described here means the oxygen dissolvingout from the quartz crucible. Almost of the oxygen, for example, 95% ofthe oxygen, becomes silicon oxide and is exhausted by the carrier gas tothe outside of the pulling chamber. Therefore, the following techniquesare known for the control of the oxygen concentration in the crystal:

Approach (1): Changing the rotational speed of the crucible so as tocontrol the supply of oxygen from the wall of the quartz crucible. Bythis technique, if the rotational speed of the crucible is increased,the amount of oxygen of the pulled monocrystal becomes higher. However,if the rotational speed of the crucible is made lower, the temperaturefluctuations of the melt become great and crystal faults become easierto occur at a low oxygen concentration. If the rotational speed of thecrucible is raised to obtain a high oxygen pulled monocrystal, it isnecessary to raise the rotational speed of the pulled crystal along withthe same. There is a problem, however, of the resonance point in thecase of pulling a pulled crystal by a wire. Further, if the rotationalspeed of the pulled crystal is made too high, deformation occurs in themonocrystal and there are problems in the control of the diameter of themonocrystal as well.

Approach (2): Control of the pressure of the carrier gas. If thepressure of the carrier gas is increased, the vaporization of thesilicon oxide is suppressed, so the amount of oxygen of the pulledmonocrystal becomes higher. However, this approach is governed largelyby the structure inside the pulling furnace, so not much can be expectedin terms of the response of the control of the oxygen concentration.

Approach (3): Spraying carrier gas on the melt silicon surface in thecrucible-so as to control the temperature of the melt silicon surfaceand control the amount of vaporization of the silicon oxide. When theheat cap is used, for example, the carrier gap between the heat cap andthe melt silicon surface and the gap between the heat cap and the pulledmonocrystal (hereinafter referred to all together as the "bottom gap")are controlled. By this technique, if the bottom gap is made smaller,the temperature of the melt surface falls, so the amount of vaporizationof the silicon oxide is held down and as a result the amount of oxygenof the pulled monocrystal becomes higher. This approach is relativelyeffective to obtain a high oxygen crystal, but if the flow of thecarrier gas is increased and the bottom gap is made too small, thecarrier gas will strike the melt hard and therefore cause bubbles in themelt. As a result, there are the problems that the crystal growth willno longer be uniform and further the variations in the oxygenconcentration in the surface will become greater.

Approach (4): Control of the discharge of the vaporized silicon oxide bythe flow of the carrier gas. If the vaporized silicon oxide isefficiently discharged by the carrier gas from the melt silicon surfaceto outside of the pulling chamber, the vaporization of the silicon oxideis promoted and as a result the oxygen concentration in the melt islowered and the amount of oxygen in the pulled crystal becomes lower.There is a gas diffusion layer of the vaporized silicon oxide directlyabove the melt surface. By using the heat cap, the flow rate of the sameis increased by the flow of the carrier gas introduced from above thepulling chamber in the narrowed gap between the heat cap and the meltsurface and therefore the thickness of the gas diffusion layer isreduced. As a result, the partial pressure of the silicon oxide on themelt surface becomes lower and vaporization of the silicon oxide ispromoted, but if a heat cap is used, an opposing phenomenonsimultaneously occurs. Further, if the crucible deforms and the meltsurface drops, the subsequent oxygen concentration in the crystal willchange.

In addition to the disadvantages in the approach (4), in the approach(3), if the size of the bottom gap is increased, the effect of theapproach (4) becomes stronger and the oxygen concentration rapidlydecreases. Therefore, to control the pulled monocrystal to within thetarget range of oxygen concentration, it is necessary to continuouslycontrol the size of the bottom gap precisely.

In this way, when using a heat sink, there is a problem that it isalways difficult to set and manage the conditions.

Further, with a silicon monocrystal used for the production of a largediameter VLSi device, it is desirable that the crystal as a whole havethe same heat history as much as possible so that the concentration ofthe oxygen taken into the crystal becomes uniform, even during thesubsequent cooling process. Therefore, a heat cap blocking the radiantheat and/or a water cooling tube are provided.

Further, there are phenomena believed to be related to the behavior ofclusters of point faults directly on the growth surface having an effecton the pressure resistance of the oxide film of the device. Therefore,Japanese Unexamined Patent Publication (Kokai) No. 3-275586 disclosesthe production of a crystal with a high oxide film pressure resistanceby lowering the pulling speed to 0.5 mm/min or less in a furnacestructure with a usual pulling speed of 1.5 mm/min. This is because itis guessed that by lengthening the residence time in the temperatureregion of over 1300° C. from the crystal growth interface, the faultsrelating to the pressure resistance of the oxide film diffuse anddisappear.

In view of these problems, the present inventor started studies from acompletely new viewpoint smashing fixed conceptions about the heat capdisclosed in U.S. Pat. No. 4,330,362 and designed to block (shield)ultraviolet rays, that is, from the viewpoint of a "carrier gas flowcontroller", and analyzed the state of flow of the carrier gas usingcomputer simulation to find a numerical solution to the Navier-Stokesequation, i.e., a non-linear fluid diffusion equation based on fluiddynamics and thermodynamics.

First, if the state of flow of carrier gas in the case of pulling amonocrystal by a pulling apparatus equipped with the heat cap disclosedin U.S. Pat. No. 4,330,362 (hereinafter referred to as a "closed typeheat cap") is considered, the result becomes as shown in FIG. 1 to FIG.3.

FIG. 1 is a view showing the state of flow of carrier gas in a pullingsystem equipped with a closed type heat cap, FIG. 2 is a view of thestate of flow of carrier gas analyzed by computer simulation of theNavier-Stokes equation, and FIG. 3 is a view of the temperaturedistribution obtained by analysis by the same computer simulation.

The heat cap 30 shown in FIG. 1 completely partitions the flow path ofthe carrier gas G into the top (shown by region X) and bottom (shown byregion Y) of a pulling chamber, so the carrier gas G introduced from thetop of the pulling chamber passes through the narrow bottom gap 33between the heat cap 30 and the pulled monocrystal 31 and surface of themelt silicon 32 to be increased in speed. By this colliding with themelt surface positioned directly under the bottom gap 33, thetemperature of the melt directly under the gap 33 falls, thevaporization of silicon oxide is suppressed, and the melt 32 of thehatched portion 34 shown in FIG. 1 becomes high in oxygen, but on theother hand, the carrier gas forcibly removes the diffusion layerincluding the silicon oxide from the melt surface, so vaporization ofsilicon oxide is promoted.

In the other area of the melt 32, however, the degree of contact withthe carrier gas G is smaller than with the portion 34 directly under thebottom gap 33, so the melt becomes relatively low in oxygenconcentration. Therefore, the distribution of the oxygen concentrationof the melt in the crucible becomes nonuniform and there is an adverseeffect on the oxygen distribution (ORG) in the silicon wafer surface ofthe pulled monocrystal 31.

Note that this state is verified by the results of computer simulationshown in FIG. 2 and FIG. 3.

This problem, it may be concluded, derives from the way the carrier gasflows. Since the heat cap partitions the pulling chamber into a top andbottom section, the carrier gas passing through the bottom gap flows ina so-called "squished" manner.

Based on these studies and the results of analyses, the present inventortook note of the "flow-control of the carrier gas" and discovered thatif the carrier gas is suitably guided in the pulling chamber, thetemperature region directly above the crystal growth interface can beexpanded, the control of the oxygen concentration and ORG can beimproved, and the condensation and falling of silicon oxide can beprevented and thereby completed the invention disclosed in JapaneseUnexamined Patent Publication (Kokai) No. 1-100,086.

A heat-cap disclosed in JPP 1-100,086 comprises a reflector body andprojected stops. The reflector consists of a tube and an inclinedcylinder provided at a lower portion of the tube with a tip (end) whichis reduced in diameter inward. The tube and the inclined cylinder may beformed integrally or together. Projections are provided at the top ofthe tube and are affixed to the top of a heat retaining member providedaround the crucible. The carrier gas is branched by the tube to flow, onone hand, through a gap between the pulling monocrystal and the tube,and, other hand, through a GaP between the tube and the heat retainingmember. That is, the heat cap can form a flow path between the tube andthe heat holding member. Thus, the heat cap can be called an open-typeheat cap.

The heat cap disclosed in JPP 1-100,086 can overcome the disadvantage ofU.S. Pat. No. 4,330,362 as a basic idea, but JPP 1-100,086 does notdisclose specific conditions. In addition, the heat cap of JPP 1-100,086requires some improvements.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a pulling system whichenables control of the oxygen concentration of the pulled monocrystal,can make the oxygen concentration (ORG) in a wafer surface uniform, canpull a monocrystal with a uniform oxygen concentration in the axialdirection, and in particular can pull a large diameter monocrystal witha high quality and a uniform heat history.

Another object of the present invention is to provide a method ofexhausting silicon oxide by a carrier gas effectively and a systemthereof.

Still another object of the present invention is to provide an improvedheat cap.

According to the present invention, there is provided a monocrystalpulling system according to the Czochralski technique, comprising: apulling chamber; a crucible provided inside the pulling chamber; aheating means provided at the circumference of the crucible and formelting a material accommodated in the crucible for forming themonocrystal; a heat retention means provided a predetermined distanceaway from the heating means and for retaining the heat of the crucible;a means for supplying a carrier gas from the top of the pulling chambertoward the surface of the melt solution in the crucible; and a flowcontroller which guides the carrier gas to the surface of the melt ofthe material forming the monocrystal and exhausts the silicon oxidevaporizing from the surface of the melt solution to the outside of thepulling chamber and which surrounds the pulled monocrystal near thesurface of the melt and is provided partially inside the crucible. Theflow controller comprises a tubular portion which has an outer diametersmaller than the inner diameter of the crucible and extendssubstantially perpendicularly along the direction of downward flow ofthe carrier gas, a constricted diameter portion which constricts indiameter from the bottom end of the tubular portion and forms a bottomgap with the pulled monocrystal, and an engagement portion whichprojects out from the top of the tubular portion and forms a top gap atthe outer circumference of the tubular portion of said flow controllerby supporting the flow controller partially in the pulling chamber. As aresult, a first flow path through which the carrier gas flows toward thebottom gap is defined between the inside of the tubular portion and thepulled up monocrystal, and a second flow path is defined comprised of aflow path of the carrier gas passing through the top gap and a flow pathof the carrier gas passing from the first flow path through the bottomgap and then passing between the surface of the silicon melt solutionand the flow controller. The silicon oxide is exhausted together withthe carrier gas through the second flow path to the outside of thepulling chamber.

Preferably, the bottom gap and the top gap are formed so that the amountof the carrier gas flowing through the bottom gap becomes greater thanthe amount of the carrier gas flowing through the top gap.

Specifically, the engagement portion is affixed to the top of a heatretaining tube provided at the outer circumference of the crucible.

Preferably, a heating means for heating the crucible is arranged betweenthe crucible and the heat retaining tube, a gap is formed between theheating means and the heat retaining tube, the gap is connected to thesecond flow path, and the silicon oxide is exhausted together with thecarrier gas to the outside of the pulling chamber through the gap.

Preferably, the area of the opening of the constricted diameter portionis 1.5 to 2.0 times the sectional area of the pulled monocrystal.

Also, preferably, the sectional area (Ru) of the top gap is 0.5 to 1.4times the sectional area (Rd) of the bottom gap.

Preferably, the top gap is positioned at least partially to the insideof the top end of the crucible.

Preferably, the flow controller is made of carbon.

Further preferably, the surface of said carbon flow controller iscovered with silicon carbide.

The length of the tubular portion of said flow controller is greaterthan the range of vertical movement of said crucible.

Preferably, the tubular portion, the constricted diameter portion, andthe engagement portion are integrally formed, or,

the tubular portion and the constricted diameter portion are integrallyformed and the engagement portion is attached detachably to the tubularportion.

Preferably, provision is made, in the pulling chamber at the top of theflow controller, of a cooling means for cooling the pulled monocrystaland introducing the carrier gas into the pulling chamber and

a peephole is provided at the outer wall of the pulling chamber on theline connecting the gap between the front end of the cooling means andthe front end of the top of the tubular portion of said flow controllerand the bottom gap.

Also, according to the present invention, there is provided a siliconoxide exhaust method for guiding a carrier gas, supplied from the top ofa pulling chamber for pulling up a monocrystal according to theCzochralski technique, to a surface of a melt of a material for forminga monocrystal and exhausting to the outside of the pulling chamber asilicon oxide vaporized from the surface of the melt accommodated in acrucible. The silicon oxide exhaust method characterized by

defining by a carrier gas branching means a bottom gap of apredetermined size between a circumference of the pulled monocrystal andthe surface of the melt and defining a top gap between the crucible anda heat retaining tube provided at the outside of the same,

defining a first flow path through which the carrier gas flows towardthe bottom gap between the carrier gas branching means and pulledmonocrystal,

defining a second flow path comprised of a flow path of the carrier gaspassing through the top gap and a flow path of said carrier gas passingfrom the first flow path through the bottom gap and then passing betweenthe surface of the silicon melt and a flow controller,

forming the bottom gap and the top gap so that the amount of the carriergas flowing through the bottom gap becomes greater than the amount ofthe carrier gas flowing through the top gap, and

exhausting said silicon oxide together with said carrier gas throughsaid second flow path to the outside of the pulling chamber.

Preferably, the crucible is heated by a heating means arranged betweenthe crucible and the heat retaining tube and a gap is formed between theheating means and the heat retaining tube, the gap is connected to thesecond flow path, and the silicon oxide is exhausted together with thecarrier gas to the outside of said pulling chamber through the gap.

Preferably, the diameter of the carrier gas branching means near thesurface of the melt solution is constricted and the area of the openingof the constricted diameter portion is 1.5 to 2.0 times the sectionalarea of the pulled monocrystal.

Also, preferably, the sectional area (Ru) of said top gap is 0.5 to 1.4times the sectional area (Rd) of the bottom gap.

Preferably, the top gap is positioned at least partially to the insideof the top end of the crucible.

According to the present invention, there is provided a monocrystalpulling system, further provision is made of a rotational control meansfor controlling the relative rotational speed of the crucible or thepulled monocrystal so as to control the concentration of oxygen includedin the monocrystal.

Further, according to the present invention, there is provided amonocrystal pulling method, wherein the relative rotational speed of thecrucible or the pulled monocrystal is controlled so as to control theconcentration of oxygen included in the monocrystal.

To pull a large diameter and high quality monocrystal, it is necessaryto (1) adjust the rotational speed of the crucible to regulate theamount of oxygen entering the melt from the wall surface of thecrucible, (2) change the rotational speed of the crucible in accordancewith changes in the area of the crucible wall surface so as to controlthe distribution of the oxygen concentration of the resultantmonocrystal to be uniform, and (3) raise the temperature of the carriergas removing the silicon oxide from the melt surface to reduce the dropin temperature at the crystal growth interface and control the radiantheat received by the crystal so as to establish a uniform heat historyenvironment.

When pulling the monocrystal, however, the silicon oxide which condensesat the top of the pulling apparatus silicon oxide. The atmosphereincluding the silicon oxide is led uniformly outside by the aspirationeffect of the large energy carrier gas passing through the outer portionof the flow controller (second flow path).

Consequently, by uniformly exhausting the silicon oxide vaporizing fromthe melt surface by a smaller amount of gas, the distribution of theoxygen concentration at the melt surface is maintained uniform, thecooling of the crystal growth surface is reduced, and as a result thepulled monocrystal is given a uniform distribution of the oxygenconcentration and becomes high in quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features and other objects and features of thepresent invention will be more apparent with reference to theaccompanying drawings, in which:

FIG. 1 is a sectional view for explaining the flow of carrier gas in aconventional pulling system provided with a closed type heat cap;

FIG. 2 is a view of the state of flow showing the results of analysis bycomputer simulation of the state of flow of the carrier gas using theNavier-Stokes equation for the same conventional pulling apparatusprovided with a closed type heat cap;

FIG. 3 is a view of the temperature distribution and at the crystalsurface and falls into the melt becomes a cause of ruin of themonocrystal, so it is critical to exhaust the vaporized silicon oxide bythe carrier gas out of the system quickly and smoothly. In the presentinvention, by providing a suitably constructed flow controller, theoptimal flow of carrier gas is realized, the amount of vaporization ofthe silicon oxide is made constant, and the particles of the siliconoxide pass between a graphite susceptor and heat shield and between theheat shield and heater and are exhausted by a vacuum pump withoutcondensing and solidifying.

That is, the carrier gas does not flow in a squished manner as in theconventional pulling apparatus. Rather, the aspiration effect of thecarrier gas passing through the second flow path formed at the outerportion of the flow controller is utilized and the carrier gas thatpasses through the inside of the flow controller and sweeps up andexhausts the atmosphere including the silicon oxide particles from themelt surface is used to draw this to the outer portion of the crucible.

Therefore, the carrier gas flowing down along the inner portion of theflow controller (first flow path) is heated by the carbon flowcontroller, passes through the gap, then is led to the melt surfacewithout excessively cooling the melt, and promotes the vaporization ofthe showing the results of analysis by computer simulation of thetemperature distribution of the carrier gas using the Navier-Stokesequation for the pulling apparatus shown in FIG. 2;

FIG. 4 is a lateral sectional view showing the pulling apparatus as awhole of a first embodiment of the present invention;

FIG. 5 is a perspective view showing a flow controller according to theembodiment;

FIG. 6 is a sectional view along line A--A of FIG. 5;

FIG. 7 is a perspective view showing a flow controller according to asecond embodiment;

FIG. 8 is a sectional view along line B--B of FIG. 7;

FIG. 9 is a sectional view showing the pulling system according to anembodiment of the present invention, which shows the dimensionalrelationship of the different parts;

FIG. 10 is a sectional view for explaining the flow of a carrier gas inthe present invention;

FIG. 11 is a view of the state of flow showing the results of analysisby computer simulation of the state of flow of a carrier gas using theNavier-Stokes equation in the pulling system of the present invention;

FIG. 12 is a view of the temperature distribution showing the results ofanalysis by computer simulation of the temperature distribution of thecarrier gas using the Navier-Stokes equation for the pulling system ofFIG. 11;

FIG. 13 is a view of the state of flow showing the results of analysisby computer simulation of the state of flow of the carrier gas using theNavier-Stokes equation for a comparative example of the pulling systemshown in FIG. 11;

FIG. 14 is a view of the temperature distribution showing the results ofanalysis by computer simulation of the temperature distribution of thecarrier gas using the Navier-Stokes equation for the pulling systemshown in FIG. 13;

FIG. 15a, 15b, 15c, and 15d are each is a graph showing the temperaturechanges at the melt surface with respect to the rotational speed of thecrucible in the pulling system of the present invention;

FIG. 16 is a graph showing the temperature distribution when thematerial of the flow controller according to the present invention ismade carbon and molybdenum; and

FIG. 17 is a graph showing the branching of the carrier gas with respectto the ratio of areas of the top gap and bottom gap according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pulling system of the present embodiment, as shown in FIG. 4,provides a quartz crucible 3 in a pulling cheer 1. The quartz crucible 3is attached to a rotatable bottom shaft 9 through a graphite susceptor8. Further, around the quartz crucible 3 is provided a heater 10 forheating and controlling the temperature of the silicon melt M in thequartz crucible 3. Between the heater 10 and the pulling chamber 1 isprovided a heat retaining tube 11.

At the top surface of the heat retaining tube 11 is attached an annularsupporting member 12. By placing an engagement portion 7 of a flowcontroller 2 on the supporting member 12, the flow controller 2 isaffixed inside the pulling chamber.

Note that reference numeral 13 denotes a cooling tube for cooling thepulled monocrystal, and reference numeral 14 denotes a peephole forobserving the surface of the melt M and the top end edge of the quartzcrucible 3.

As shown in FIG. 5, FIG. 6, and FIG. 9, the flow controller 2 accordingto the present embodiment has an outer diameter d smaller than the innerdiameter D of the quartz crucible 3 and is comprised of a tubularportion 4 which extends substantially perpendicularly along thedirection of downward flow of the carrier gas G and a constricteddiameter portion 6 which constricts in diameter gradually from thebottom end of the tubular portion 4 and forms the small bottom gap 5with the pulled monocrystal S.

The tubular portion 4 is particularly effective in the case of pullingthe monocrystal by the batch-type CZ technique. Namely, the height ofthe tubular portion 4 in the present embodiment is set so that the areaof the opening of the opening 17 becomes constant even if the graphitesusceptor 8 is moved up and down between the limit of descent and thelimit of rise. Therefore, even if the graphite susceptor 8 rises alongwith the pulling of the monocrystal, the area of the opening 17 can beheld constant, so there is no fluctuation in the flow of the carrier gasG mentioned later.

At the bottom end of the constricted diameter portion 6, an opening 16is made. This has an opening diameter B in accordance with the diameterA of the pulled monocrystal S and in consideration of the area of thebottom gap 5. With an opening area (=πB² /4) less than 1.5 times thecrystal area, if the crystal suspended by the wire deviates off center,it may contact the flow controller 2. On the other hand, if greater than2.0 times, the flow rate of the gas on the melt will fall. Therefore,the area of the opening at the front end of the constricted diameterportion (lateral sectional area) is preferably 1.5 to 2.0 times the areaof the pulled monocrystal S (lateral sectional area).

By way of note, the above numerical figures limit the ratio between thearea of the opening at the front end of the constricted diameter portionand the lateral sectional area of the monocrystal, but in the flowcontroller according to the present invention, the same effect isobtained even if the ratio between the area of the bottom gap and thelateral sectional area of the monocrystal is limited. The ratio in thiscase is 0.5 to 1.0.

The flow controller 2 must withstand high temperatures and not dischargeany heavy metal elements since it is directly above the melt. Use ismade of a refractory metal like molybdenum(Mo) or tungsten(W) orcarbon(C). A refractory metal reflects radiant heat and has a largeshielding effect, while carbon absorbs radiant heat and converselydischarges radiant heat to the crystal.

FIG. 16 is a graph showing the results of measurement of the temperaturefor comparing the effects of radiant heat of molybdenum and carbon at aposition 100 mm off from the center of the pulling system.

As shown by this temperature distribution, if use is made of a carbonflow controller, the temperature at the crystal growth interface becomeshigh, so in the past it was considered that this was detrimental toincreasing the pulling speed, but since the crystal quality isconsidered important now, use of carbon is effective. In addition, acarbon flow controller has the effect of heating the carrier gas flowingthrough the first flow path P₁.

Further, it is preferable that the surface of the carbon flow controllerbe coated with silicon carbide(SiC). Giving this silicon carbide coatingis advantageous to the lifetime of the flow controller 2 and, further,is effective in inhibiting the discharge of the heavy metal elementsincluded in small amounts in the carbon.

The engagement portion 7 for placing the flow controller 2 on thesupporting member 12 may be formed integrally with the tubular portion 4as shown in FIG. 5 and FIG. 6. Further, as shown in FIG. 7 and FIG. 8,the engagement portion 7 may be formed separately and be affixed to thetubular portion 4 by carbon bolts 15.

In the pulling system of this embodiment, by affixing theabove-mentioned flow controller 2 inside the pulling chamber 1, the flowpaths of the carrier gas G supplied from the top of the pulling chamberI become as follows:

As shown in FIG. 10, first, as the first flow path, there is the flowpath P₁ in which the carrier gas G branches at the top end of thetubular portion 4 of the flow controller 2 and reaches the inside of thetubular portion 4. Connecting to the first flow path P₁, there is thesecond flow path P₂, in which the carrier gas G passes from the firstflow path P₁ through the gap between the bottom end of the constricteddiameter portion 6 and the pulled monocrystal S (hereinafter referred toas the "bottom gap 5") and through the gap between the bottom end of theconstricted diameter portion 6 and the surface of the melt M, thenpasses between the surface of the melt M and the flow controller 2, andfurther forms the atmosphere including the silicon oxide accompanyingthe rising air flow between the flow controller 2 and the inner surface3a of the quartz crucible.

The ratio of the carrier gas G led through the first flow path P₁ andthe second flow path P₂ is related to the areas of the top and bottomgaps, that is, the area Ru of the gap at the outer portion of thetubular portion of the flow controller 2 in the second flow path P₂(hereinafter this gap is referred to as the "top gap 18") and the areaRd of the bottom gap 5 between the constricted diameter portion 6 of theflow controller 2 and the pulled monocrystal S in the first flow pathP₁, but the carrier gas flowing through the bottom gap 5 encounters theresistance of the rising air flow from the melt, so judging from theresults of simulation of the embodiments shown in FIG. 11 and FIG. 13,when the area ratio Ru/Rd is about 0.7, the carrier gas G flows in equalamounts (branching ratio 50%) in the first flow path P₁ and the secondflow path

By way of note, if the branching ratio of the gas flowing through thefirst flow path P₁ is less than 20% of the overall amount, it is notpossible to suppress the rising air flow from the melt surface, thesilicon oxide deposits on the cooled monocrystal, and, if it condensesand falls, it will ruin the crystal. Conversely, if the branching ratioof the gas flowing through the first flow path P₁ is over 80% of theoverall amount, the atmosphere including the silicon oxide above themelt surface will not be able to be effectively exhausted. Therefore, interms of the branching ratio, the carrier gas flowing through the firstflow path P₁ preferably should be 20% to 80% of the total amount.Converting this to the area ratio Ru/Rd between the top gap 18 and thebottom gap 5, the preferable region of the ratio may be said to be 0.4to 1.4 (see FIG. 17).

The carrier gas G led into the second flow path P₂ is heated by thecarbon flow controller 2 and strikes the melt surface M withoutovercooling the crystal growth interface, replaces the atmosphereincluding the silicon oxide removed by the carrier gas from the firstflow path P₁, and diffuses over the melt surface as a whole. Thisdetermines the amount of vaporization of the silicon oxide, defines theoxygen concentration of the melt surface from which the crystal israised, and stabilizes it at the low oxygen side.

Therefore, to obtain a medium oxygen and high oxygen monocrystal, ratherthan make the bottom gap 5 narrower, the rotational speed of thecrucible is rather increased.

If the rotational speed of the crucible is increased, as clear from thetemperature change of the melt surface shown in FIG. 15, the stabilityof the temperature change increases and the stability of thesolid-liquid interface increases. In this case, in the state with noflow controller 2, if the rotation of the crystal is not increased sothat the ratio of the crystal rotation/crucible rotation becomesconstant, numerous crystal faults occur, experience shows, but with thisconstruction, it was confirmed that it was sufficient to make thecrystal rotation a maximum of 20 rpm.

Further, as the second flow path P₂, there is the flow path in which thecarrier gas G supplied from the top of the pulling chamber 1 branches atthe top end of the tubular portion 4 of the flow controller 2 and passesthrough the top gap 18 to flow to the outer portion of the tubularportion 4. The carrier gas G led to this second flow path P₂ ends upworking with the carrier gas G passing through the first flow path P₁and the other second flow path P₂ to function to exhaust the siliconoxide outside the system and, further, has the following functions:

When the carrier gas G branching at the top end of the tubular portion 4of the flow controller 2 and passing through the top gap 18 to be ledinto the second flow path P₂ flows down along the outer surface of theflow controller 2 and then flows further down along the outer surface ofthe graphite susceptor 8, that is, when passing through the opening 18between the graphite susceptor 8 and the flow controller 2, it sweeps upthe atmosphere including the silicon oxide at the outside of the flowcontroller 2 by the aspiration effect and reduces the air pressure.

Therefore, the silicon oxide vaporizing from the surface of the melt Mis exhausted outside of the quartz crucible 3 along with the largeenergy gas from the flow path P₁, so will not condense and solidify atthe top of the flow controller facing the melt and fall into the melt.As a result, the yield of the monocrystal is improved.

Since the atmosphere including the silicon oxide is exhausted in thisway from the second flow path P₂ by the aspiration effect, the carriergas G led to the first flow path P₁ uniformly contacts the entiresurface of the melt M as rectified.

Therefore, the amount of vaporization of the silicon oxide vaporizingfrom the melt surface becomes uniform and it becomes possible tosuppress the adverse effects on the oxygen concentration caused byturbulence of the carrier gas G, that is, to improve the oxygendistribution (ORG) in the wafer surface of the pulled monocrystal.

Further, the carrier gas G supplied from the top of the pulling chamber1 branches into the first flow path P₁ and the second flow path P₂ atthe top end of the tubular portion 4 of the flow controller 2, so forexample if the resistance through the first flow path P₁ fluctuates,there is a function of absorbing that fluctuation. That is, when themonocrystal S is started to be pulled, the top of the pulled monocrystalS is still not positioned at the constricted diameter portion 6 of theflow controller 2, so the gap dimension of the bottom gap 5 is large.

In this way, the flow area of the first flow path P₁ is large, so alarge amount of carrier gas G is led to the first flow path P₁. When themonocrystal S is pulled, the pulled monocrystal rises to the inside ofthe flow controller 2 and the flow area rapidly becomes smaller.

By this, the flow resistance of the first flow path P₁ rapidlyincreases, the flow rate of the carrier gas G passing through the bottomgap 5 becomes faster, and the top of the pulled monocrystal S becomesrelatively high in oxygen concentration, but in the present invention,an amount of carrier gas G equal to the increase in the flow resistanceof the bottom gap 5 is led into the second flow path P₂, so as a resultthe flow rate of the carrier gas G passing through the bottom gap 5 doesnot fluctuate that much.

When there is no branching, with a closed heat cap disclosed in U.S.Pat. No. 4,330,362, if the clearance of the bottom gap is made larger,the cooling of the melt surface decreases, vaporization of the siliconoxide is promoted, and a low oxygen concentration results. In aconstruction as with the flow controller of the present invention,however, where the gas branches to the first flow path P₁ and the secondflow path P₂, the flow rate does not fluctuate that much, so it becomesunnecessary to finely adjust the bottom gap and otherwise be strict inthe settings. Therefore, the distribution of the oxygen concentration inthe axial direction of the pulled monocrystal S becomes constant.

After this, the rotation of the crucible is adjusted to control theoxygen concentration.

However, to pull a large diameter, high quality monocrystal withoutoverly reducing the pulling speed, it is necessary to lengthen theregion of over 1300° C. near the crystal growth interface. To achievethis, it is advantageous to reduce the flow of the carrier gas or toraise the temperature. Therefore, in the present invention, an effectcan be expected by having the radiant heat from the melt received by thecarbon flow controller 2 and re-radiated to the pulled monocrystal.Further, after passing this region, it is possible to efficientlyproduce the high quality monocrystal by raising the cooling ability ofthe pulled monocrystal by the top water cooling tube.

In addition to this, the melt surface near the inner peripheral wall ofthe quartz crucible 3 is not covered by the flow controller 2, so byobserving the melt surface from a peephole 14 etc. provided in thepulling chamber 1, it is possible to quickly deal with any heatdeformation in the top end of the quartz crucible 3, recrystallizationor silicon deposition near the inner peripheral wall of the quartzcrucible 3, or other problems when they occur.

Further, when pulling a monocrystal by the batch type CZ technique, thegraphite susceptor 8 on which the quartz crucible 3 is carried is raisedin accordance with the pulling of the monocrystal S so as to maintainthe dimensions of the bottom gap. In the flow controller 2 of thepresent embodiment, a carbon flow controller 2 absorbs the heatirradiated from the melt surface, then radiates that heat to the pulledmonocrystal. Considering this, the tubular portion 4 is formed in theflow controller 2 so that the temperature will not fall to the extentwhere silicon oxide condenses. Therefore, even if the quartz crucible 3and the graphite susceptor 8 are raised, the gap formed between thetubular portion 4 and the inner surface of the quartz crucible 3, thatis, the area of the opening of the opening 18, can be maintainedconstant. Consequently, even if the quartz crucible 3 rises, there is nofluctuation caused in the flow of the carrier gas G passing through.

The present invention will be explained in further detail to help theeffect of the flow controller to be understood.

FIG. 17 is a graph showing the branching of the carrier gas with respectto a ratio Ru/Rd of the bottom gap area Rd and the top gap ratio Ru. Inthe figure, when the area ratio Ru/Rd is 0.4, the flow of the first flowpath becomes 20% while when it is 1.4, the flow of the first flow pathbecomes 80%.

At this time, if the area ratio Ru/Rd is smaller than 0.4, the crystalis overcooled and a high quality crystal cannot be obtained. Further, ifthe area ratio Ru/Rd is over 1.4, the carrier gas flowing through thefirst flow path becomes too little, so the gas including silicon oxidefrom the bottom gap rises and adheres to the top of the crystal. Thiscondenses and falls into the melt, thereby creating the problem of theruining of the monocrystal.

This relationship will be explained with reference to a pullingapparatus for a 6 inch crystal.

First, the crystal diameter A is 156 mm and the constricted diameterportion opening B of the flow controller is 210 mm, so the ratio of thearea of the opening to the area of the crystal becomes 1.8.

Further, the area Rd of the bottom gap after the pulled monocrystalpasses through the opening of the constricted diameter portion of theflow controller becomes 15,523 mm². The diameter d of the tubularportion of the flow controller is 342 mm, the opening width C is 12 mm,and the width of the engagement portion 7 is 60 mm, so the area of thetop gap 18 is 10,465 mm². Therefore, the area ratio Ru/Rd becomes 0.67.Using such a pulling system, monocrystals were produced under conditionsof 40 nl/min of argon gas and 10 to 20 Torr of vacuum. The standarddeviations in the target of the oxygen concentrations were as shown inTable 1. It was possible to obtain the monocrystals aimed for in thepresent invention.

                  TABLE 1                                                         ______________________________________                                        Standard Deviation in Oxygen Concentrations                                               Target               Standard                                     Class       value        Power   deviation                                    ______________________________________                                        Low oxygen  1.15 × 10.sup.18                                                                     121     0.062                                        Medium oxygen                                                                             1.35 × 10.sup.18                                                                     221     0.044                                        High oxygen 1.55 × 10.sup.18                                                                     110     0.039                                        ______________________________________                                    

The embodiments explained above were described to facilitate theunderstanding of the present invention and were not meant to limit thepresent invention in any way. Therefore, the elements disclosed in theabove embodiment include all design modifications and equivalentsfalling under the technical scope of the present invention.

As explained above, according to the present invention, control of theconcentration of oxygen in the pulled monocrystal is possible,achievement of a uniform oxygen distribution (ORG) in the wafer surfaceis possible, it is possible to pull a monocrystal with a uniform oxygenconcentration with respect to the axial direction, and it is possible topull a high quality monocrystal having a particularly large diameter.

We claim:
 1. A silicon oxide exhaust method for guiding a carrier gas,supplied from the top of a pulling chamber for pulling up a monocrystalaccording to the Czochralski technique, to a surface of a melt of amaterial for forming a monocrystal and exhausting to the outside of saidpulling chamber silicon oxide vaporized from the surface of the meltaccommodated in a crucible,said silicon oxide exhaust method comprisingdefining by a carrier gas branching means a bottom gap between acircumference of the pulled monocrystal and the surface of said melt anddefining a top gap between said crucible and a heat retaining tubeprovided at the outside of the same, defining a first flow path throughwhich said carrier gas flows toward said bottom gap between said carriergas branching means and pulled monocrystal, defining a second flow pathcomprised of a flow path of the carrier gas passing through said top gapand a flow path of said carrier gas passing from said first flow paththrough said bottom gap and then passing between the surface of saidsilicon melt and a flow controller, forming the bottom gap and the topgap so that the amount of the carrier gas flowing through said bottomgap becomes greater than the amount of the carrier gas flowing throughsaid top gap, and exhausting said silicon oxide together with saidcarrier gas through said second flow path to the outside of the pullingchamber.
 2. A silicon oxide exhaust method as set forth in claim 1,whereinsaid crucible is heated by a heating means arranged between saidcrucible and said heat retaining tube and a gap is formed between saidheating means and said heat retaining tube, said gap is connected tosaid second flow path, and said silicon oxide is exhausted together withsaid carrier gas to the outside of said pulling chamber through saidgap.
 3. A silicon oxide exhaust method as set forth in claims 2, whereina portion of said top gap is positioned to the inside of the top end ofthe crucible.
 4. A silicon oxide exhaust method as set forth in claim 1,whereina diameter of the carrier gas branching means near the surface ofthe melt solution is constricted and an area of the opening of theconstricted diameter portion is 1.5 to 2.0 times the sectional area ofthe pulled monocrystal.
 5. A silicon oxide exhaust method as set forthin claim 1, wherein a sectional area (Ru) of said top gap is 0.5 to 1.4times the sectional area (Rd) of the bottom gap.
 6. A monocrystalpulling method as set forth in claims 1, wherein a relative rotationalspeed of the crucible or the pulled monocrystal is controlled so as tocontrol the concentration of oxygen included in said monocrystal.